An optical waveguide device using an electrooptical crystal such as a LiNbO3 (LN) substrate, a LiTaO3 substrate or the like is being developed. These optical waveguide devices may be formed through processes of providing an electrode near an optical waveguide after forming the optical waveguide by forming a metal film such as Ti on a part of a crystal substrate and thermally diffusing the metal film or by proton-exchanging in benzoic acid after patterning. Japanese Patent Application Publication No. 05-53086 discloses a Mach-Zehnder optical modulator as the optical waveguide device.
The Mach-Zehnder optical modulator may have a Y-shaped splitter after a curved waveguide. In the structure, the splitter receives a mode shifted in the curved waveguide. Thus, a branching ration may be shifted from 50%. This may result in degradation of an extinction ratio of the Mach-Zehnder. A straight waveguide may be provided before the splitter in order to move the branching ratio closer to 50%. However, in this case, there is a problem that the optical modulator grows in size.